1. Field of the Invention
The present invention relates to a solid electrolytic capacitor, and particularly to a method for manufacturing a solid electrolytic capacitor using a sintered body of valve metal.
2. Description of the Related Art
Solid electrolytic capacitors employ, as an anode, valve metal such as tantalum, niobium, and aluminum. Solid electrolytic capacitors represented by tantalum capacitors, niobium capacitors use a high melting point material as a valve metal. In these capacitors, anode members are fabricated by sintering a pressed compact of valve metal powders
FIG. 4 is typical example of such capacitors. In FIG. 4, a solid electrolytic capacitor 100 has an anode body 1 in which an anode lead 2 is embedded, a solid electrolyte layer 4 formed on an oxide film layer 3 on the surface of the anode body 1, a graphite layer 5, and a silver layer 6, formed over the surface of the solid electrolyte layer 4. The anode lead 2 is connected with an anode terminal 9, and the silver layer 6 is fixed, through a silver adhesive layer 7, to a cathode terminal 10. The entire body is covered with a resin mold 8 with the anode and cathode terminals 9, 10 exposed to the bottom the capacitor.
An anode body used in such a solid electrolytic capacitor is formed through various steps. Referring to a process flow diagram in FIG. 3, valve metal powders are mixed and granulated with organic binder in order to improve fluidity of the powders in press-molding valve metal powders and adhesiveness among the powders after press-molding (at S1). Next, the granulated powders are press-molded to produce a pressed compact thereof (at S2). The compact is sintered in vacuum to form a sintered body (at S3), and then a dielectric oxide film layer is formed through anodization to provide the anode body (at S4). Further, to complete a solid electrolytic capacitor, a solid electrolyte is formed on the dielectric oxide film layer of the anode body (at S5). Subsequently, a graphite layer is formed on the solid electrolyte (at S6) and a silver layer is formed on the graphite layer (at S7). Then, electrical connections are made between the silver layer and a cathode terminal and between an anode lead and an anode terminal (at S8), and an overmolding process follows (at S9).
Organic binder used in the granulation step is generally removed by thermal decomposition and flying in all directions in the sintering step in vacuum. However, in this removal process, residuals of the organic binder are easy to remain on the sintered body. In the case where binder residuals remain on the sintered body of valve metal, impurities such as charred metals are formed in an oxide film formed thereafter, and a problem arises that such impurities cause leakage current in a completed capacitor.
Typically, solid electrolytic capacitors having, for example, a rated voltage of 10V and a capacitance of 10 μF show a leakage current of 10 μA or less. However, further reduction in leakage current is required. Typically, 1 μA or less is preferred.
Japanese Unexamined Patent Application Publication (JP-A) No. 2004-335630 discloses a method for manufacturing a solid electrolytic capacitor in which the binder residual concentration of the sintered body is reduced to suppress leakage current. According to the method, water soluble solid binder and organic solvent soluble binder are used as binders for the granulation process. In order to remove the binders from a shaped form, two dissolution cleaning processes of organic solvent immersion and warm pure water immersion are performed in series between the press-molding and the sintering.